Remove incomplete row in chapter 8

[DanielYang59]

book git:(master)   export PATH="$(ruby -e 'puts Gem.bindir'):$PATH"
  asciidoctor --version
  make html
Asciidoctor 2.0.26 [https://asciidoctor.org]
Runtime Environment (ruby 4.0.6 (2026-07-14 revision 03b6d3f889) +PRISM [arm64-darwin25]) (lc:UTF-8 fs:UTF-8 in:UTF-8 ex:UTF-8)
asciidoctor \
                -a stylesheet=theme/asciidoctor-clean.custom.css \
                -a source-highlighter=pygments \
                -a pygments-style=friendly \
                -a '!example-caption' \
                -a sectanchors \
                *.asciidoc
asciidoctor: ERROR: chapter_08_events_and_message_bus.asciidoc: line 818: dropping cells from incomplete row detected end of table
asciidoctor: ERROR: chapter_08_events_and_message_bus.asciidoc: line 818: dropping cells from incomplete row detected end of table